Hydrocephalus is a defective condition of the brain caused by an imbalance between production of cerebral spinal fluid (CSF) within the brain and the capacity of the brain to re-absorb such fluid at normal pressure. Hydrocephalus may be congenital, accidental or age related and can result in loss of a wide range of physical and mental faculties. The accepted method of treatment of the condition is to divert excess fluid which the brain, through its impairment, is unable to absorb, to some other part of the body such as the right atrium or the peritoneal cavity, where the fluid can re-enter the blood stream. The primary technical challenge associated with this method of treatment is developing the capacity to control the conditions of pressure and flow within the brain in such a manner as to enable lost faculties to be restored, depending on the severity of the condition. This challenge is exacerbated by the greatly differing range of impairment between different patients.
Various devices have been developed to control the pressure and flow conditions within the brain to desired levels. Such devices are commonly known as valves. If the flow imbalance referred to above is not addressed, pressure within the brain rises to an abnormal level. Such increased pressure causes the ventricles of the brain to expand and causes abnormal stress and damage within the brain tissue. Consequently, many valve devices focus on a technology that controls the pressure within the ventricles of the brain directly, restricting that pressure to a desired level and allowing flow rates to vary to accommodate the target pressure. Such devices are referred to as pressure control valves and commonly comprise an orifice that may be forced open by fluid pressure within the brain against some form of resistant mechanism such as a ball and spring or a slit in a tubular member. The majority of valves in use at the present time are pressure control valves. Difficulties with this type of valve arise in being able to set the spring pressure accurately. In addition, the valves are susceptible to malfunction when subject to pressure waves which occur in the brain and they are also very sensitive to changes in pressure due to body posture.
An alternative type of valve directly controls the amount of fluid diverted. Control is effected in such a manner that the desired pressure conditions within the brain are achieved as a secondary effect. Such devices are referred to as flow control valves. Currently available valves control flow by some form of restrictive orifice, which in some cases can be varied to give different flow rates. However, at the pressures existing in the brain (around 12 cm H2O in a normal supine person), and at the very low diverted flow rates involved (some fraction of 0.35 ml/min) the size of orifice is minute, and it is difficult to predetermine accurately the resulting flow.